While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration densities and operating speeds in LSI devices, DUV and EUV lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is thought requisite to the micropatterning technique capable of achieving a feature size of 0.13 μm or less.
The ArF lithography started partial use from the fabrication of 130-nm node devices and became the main lithography since 90-nm node devices. Although lithography using F2 laser (157 nm) was initially thought promising as the next lithography for 45-nm node devices, its development was retarded by several problems. A highlight was suddenly placed on the ArF immersion lithography that introduces a liquid having a higher refractive index than air (e.g., water, ethylene glycol, glycerol) between the projection lens and the wafer, allowing the projection lens to be designed to a numerical aperture (NA) of 1.0 or higher and achieving a higher resolution. While the ArF immersion lithography has entered the commercial stage, the technology still needs a resist material which is substantially non-leachable in water.
In the ArF lithography (193 nm), a high sensitivity resist material capable of achieving a high resolution at a small dose of exposure is needed to prevent the degradation of precise and expensive optical system materials. Among several measures for providing high sensitivity resist material, the most common is to select each component which is highly transparent at the wavelength of 193 nm. For example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating copolymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers have been proposed as the base resin. This choice is effective to some extent in enhancing the transparency of a resin alone.
Studies have also been made on photoacid generators (PAGs) and diffusion inhibitors. Sulfonium salts such as triphenylsulfonium nonafuorobutanesulfonate are typically used as the PAG because of stability in resist compositions. Amines are typically used as the diffusion inhibitor. Many problems associated with line width roughness (LWR) as an index of pattern roughness and pattern profile are left unsolved. Also use of weak acid onium salts as the diffusion inhibitor is under study. For example, JP-A H11-125907 describes that patterns with minimal roughness can be formed using a compound capable of generating a carboxylic acid having a boiling point of at least 150° C. JP-A H11-327143 reports improvements in sensitivity, resolution and exposure margin by the addition of sulfonic acid ammonium salts or carboxylic acid ammonium salts. Also, JP-A 2001-281849 describes that a resist composition for KrF or EB lithography comprising a PAG capable of generating a fluorinated carboxylic acid is improved in resolution and process latitude such as exposure margin and depth of focus. Further, JP 4226803 describes a positive photosensitive composition for ArF excimer laser lithography comprising a carboxylic acid onium salt. These systems are based on the mechanism that a salt exchange occurs between a weak acid onium salt and a strong acid (sulfonic acid) generated by another PAG upon exposure, to form a weak acid and a strong acid onium salt. That is, the strong acid (α,α-difluorosulfonic acid) having high acidity is replaced by a weak acid (alkanesulfonic acid or carboxylic acid), thereby suppressing acid-aided decomposition reaction of acid labile group and reducing or controlling the distance of acid diffusion. The onium salt apparently functions as a quencher, that is, diffusion inhibitor. However, as the microfabrication technology is currently further advanced, the resist compositions using such weak acid onium salts become unsatisfactory with respect to resolution, roughness, depth of focus and the like, particularly when processed by the ArF immersion lithography. The alkanesulfonic acid salts have a low quencher capability because the acidity is not fully low. The carboxylic acid salts are not only insufficient in the above-referred properties, but also suffer from a leaching problem because they are highly hydrophilic. That is, the salts can be leached in immersion water used in the ArF immersion lithography. Since this leaching has a concern of contaminating the exposure tool and can also cause defects, it is desired to minimize the leaching.
JP-A 2012-108447 discloses an onium salt of fluoroalkanesulfonamide as the weak acid onium salt. When this onium salt is applied to the upcoming generation of ultrafine processing using ArF lithography or ArF immersion lithography, the line width roughness (LWR), indicative of pattern roughness, and resolution are yet short. There is still a need for a weak acid onium salt having improved quencher function.